For many years, conventional American vehicle engine suspension systems have employed 12 Hz vertical frequency suspension components (i.e., engine mounts). This frequency has been practical since it is typically low enough to isolate engine firing disturbances, but at the same time is high enough to facilitate a compact elastomeric design. A lower frequency cannot always be used due to the fact that static deflection, and static part strain for a given envelope size, decrease as a function of frequency squared. In a linear system, for example, this relationship is expressed mathematically as: EQU .delta.=9.788/f.sub.n.sup.2,
where:
.delta.=static deflection in inches; and PA1 f.sub.n =vertical natural frequency of the mounting in Hertz.
Hence, as the mounting is designed toward lower natural frequencies, the static deflection increases as a square function (e.g., to reduce f.sub.n by two requires static deflection increase by four). This means that for a given mounting envelope size, reduction of actual frequency results in a penalty in terms of part strain and life.
Another factor complicating the design of relatively low-frequency engine suspension systems is mount stability. Designs that carry weight in shear tend to be large. On the other hand, designs that carry weight in compression tend to be laterally unstable. The traditional solution to this problem is to use a design that carries the weight in a combination of shear and compression. This has resulted in "V-type" mounting arrangements or "conical" mountings, well known in the art.
In European vehicle engine suspension systems, lower frequency engine mounts (typically 8 Hz) are more common. This is due to several factors. Among these are lower engine idle speeds, light-weight truck structures and differences in Government regulations. Relatively large engine mounts can be and are employed in many European vehicles since they are designed to accommodate mounts of large size. However, for the many other vehicles which have not been so designed, more compact low-frequency mounts are required.